Human placental estradiol 17 beta-dehydrogenase has been purified and crystallized and attempts are being made to grow crystals large enough to allow structural studies with X-ray crystallography and compare these with results obtained using affinity-labeling steroids. Affinity-labeling studies of serum guinea pig progesterone binding protein will be carried out with bromoacetoxyprogesterone derivatives. There appear to be adequate amounts of estradiol 17 beta-dehydrogenase and estradiol 17 alpha-dehydrogenase activity in horse placenta (when it is collected in a 20 percent glycerol buffer) to allow further purification to homogeneity. The enzyme is microsomal and is solubilized octylglycoside. Our plan is to separate microsomes, extract the activities with octylglycoside and purify them with affinity-chromotography for ultimate structural studies in comparison with the enzyme from human placenta.